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140 IBM System Storage DS6000 Series: Copy Services with IBM System z
Figure 13-4 Consistency Group: Example 3
In this case, the Metro Mirror target volumes reflect only the first and third write operation, not
including the second operation.
Typical database management software can recover its databases by using its log files if
dependent write operations are kept. At the same time, you should not need to care whether
independent write operations are kept in sequence.
13.3.3 FREEZE and RUN parameters
Messages and environmental triggers related to Metro Mirror problems can be detected by
your automation software. You can then use the TSO command CGROUP with the FREEZE
parameter (or equivalent command through the other interfaces) to suspend all required
secondary Metro Mirror operations.
The FREEZE option removes the Metro Mirror paths, and suspends the volume pairs for a
given LSS pairing. If used in combination with the CGROUP(Y) option of the CESTPATH
command, then an
extended long busy (for z/OS) or queue full (for open systems) condition
will be forced on all the LSSs where the FREEZE is issued, and thus, all the I/Os to the
respective primary volumes will be queued while in the extended long busy window. Once
your automation process detects that the updates can be resumed, it can issue the CGROUP
command with the RUN parameter. This command releases all I/O to the primary devices,
but still leaves the secondary devices suspended at a point-in-time (with the FREEZE the
associated paths are removed).
The default 2-minute timer of the extended long busy state gives the automation enough time
to issue a CGROUP FREEZE command to the necessary LSSs. I/O resumes after the default
2 minutes if a CGROUP RUN command is not received.
1st
2nd
3rd
independent
write
operations
Application
on
Servers
Completed
Wait
Wait
LSS11
LSS12
LSS13
LSS21
LSS22
LSS23
Source DS6000 Target DS6000
Completed